For applications which do not require high dynamics, speed-controlled asynchronous machines are operated on frequency converters. Proceeding from a nominal point of the asynchronous machine, a ratio of phase voltage U.sub.1 to stator frequency f.sub.1 is kept constant as the speed decreases, as is evident from the characteristic curve in FIG. 1. The result is a magnetization current I.sub.m that is constant at each speed n and also independent of load. The nominal point of the asynchronous machine is selected so that at a given intermediate circuit voltage U.sub.zw, the asynchronous machine may still absorb the nominal magnetization current. If the rotation speed n is raised above that point, the voltage U.sub.1 may not be kept proportional to the stator frequency f.sub.1, but rather remains constant. The magnetization current I.sub.m thus assumes a value proportional to U.sub.1max /n, since stator frequency f.sub.1 (ignoring the slight slippage) in turn behaves proportionally with respect to speed n, as depicted in FIG. 2 using a 1/n control curve. It should be noted with respect to this approach that it is simple, robust, and sufficient for many applications.
For highly dynamic drive systems, a field-oriented control system, which allows rapid torque adjustments, is generally accepted. In this, an excitation current is set using the U/f control principle, i.e. such that in the base speed region, the magnetization current I.sub.m is kept constant up to the cutoff point, cutoff frequency, or cutoff speed n.sub.eck. In addition, at higher frequencies the magnetization current I.sub.m is decreased in a field control speed region in accordance with the 1/n characteristic curve, as is evident from the characteristic curve shown in FIG. 2. Since both the intermediate circuit voltage and the magnetization current are constant, the cutoff point also does not change. A control mode of this kind is also the basis of the one described in an article by H. Grotstollen and J. Wiesing in ETZ, Vol. 115 (1994) No. 9, pp. 486-491, entitled "Betrieb Der Asynchronmaschine Im Feldschaechbereich."
Applications of adjustable-speed drives such as those in electric vehicles require the maximum possible utilization of the available energy at every operating point. In the base speed region (i.e. for speeds below the nominal speed or cutoff speed n.sub.eck), the magnetization current I.sub.m control function described above results in a low efficiency under partial load.
In the base speed region, the optimum distribution of the stator current I.sub.1 into magnetization current I.sub.m and rotor current I.sub.2 may be calculated for each value of the stator current I.sub.1. For a given stator current I.sub.1, this distribution may be established by way of the rotor frequency f.sub.2. The pertinent value pairs in each case may be represented in tabular form or in a control characteristic curve such as the one shown in FIG. 3. With this control method, the operating point is thus defined via the stator current I.sub.1 and rotor frequency f.sub.2 using a current-slip method.
With this method as well, the stator voltage U.sub.1 rises as the current and the speed increase. Maximum stator voltage is reached at the cutoff speed n.sub.eck. This cutoff speed n.sub.eck depends, however, on the magnitude of the stator current I.sub.1, since for a lower stator current I.sub.1 the corresponding magnetization current I.sub.m is also lower, and the induced stator voltage U.sub.1 is correspondingly lower as well. At a lower stator current I.sub.1 the voltage limit is thus not reached until a higher speed n, as is apparent from the characteristic curve in FIG. 4.
When the asynchronous machine is used for drive systems of electric vehicles, the cutoff speed is also determined by a battery voltage. The battery voltage fluctuates depending on a charge state and the load. Minimum operating voltages that are typical for modern traction batteries, with no-load voltages of 300 V, are approximately 200 V for drive and up to 400 V for regeneration. Corresponding to these different voltage values, a different cutoff speed n.sub.eck occurs in each case.
For operation beyond the voltage limit curve, torque- or efficiency-optimized operation is achieved in each case with utilization of the maximum voltage.
In the case of the current-slip control system, there thus exists the object of establishing the rotor frequency f.sub.2 which is appropriate for the respective current at the instantaneous voltage of the battery or the intermediate circuit.
In a control system applied in a VW hybrid, this object is achieved by an electronic circuit. This electronic control circuit, however, allows time constants to be adjusted only by changing components. This is therefore very rigid or cumbersome, and thus does not promote utilization in a vehicle.
A way is therefore being sought to replace this control circuit with a method that is flexible, economical, and easy to implement, if possible using components which are already available.